It's a tween dream! Miley Cyrus and Taylor Swift joined Selena Gomez at Conway Studios yesterday, for what appears to be a top-secret project ...

The American Music Awards are tonight -- maybe they're doing a quickie track for the show?

Miley joined Selena at the recording studio around 2pm Saturday and stayed for about two hours. Then Taylor arrived around 7pm and she and Selena left for dinner at Osteria La Buca in West Hollywood. After, they both went back to the studio and Taylor stayed for another hour.

We all know what happened next ... Justin Bieber arrived to join his girlfriend before heading back to his hotel together to spend their second night in a row together!